Messages in this thread |  | | | Date | Tue, 19 May 2009 20:51:40 +0200 | | From | Oleg Nesterov <> | | Subject | Re: INFO: possible circular locking dependency at cleanup_workqueue_thread |
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On 05/19, Johannes Berg wrote: > > On Tue, 2009-05-19 at 18:09 +0200, Oleg Nesterov wrote: > > > > Right. But exactly this happens in the hibernate case -- > > > > not sure I understand your "exactly this" ;) > > > > But your explanation of the deadlock below looks great! > > Yeah... I got side-tracked, I had a scenario in mind that actually > needed cpu_add_remove_lock(). > > > except I don't understand how cpu_add_remove_lock makes the difference... > > And thus I can't understand why cpu_down() (called lockless) have the > > same problems. Please see below. > > > > > Anyway, you can have a deadlock like this: > > > > > > CPU 3 CPU 2 CPU 1 > > > suspend/hibernate > > > something: > > > rtnl_lock() device_pm_lock() > > > -> mutex_lock(&dpm_list_mtx) > > > > > > mutex_lock(&dpm_list_mtx) > > > > > > linkwatch_work > > > -> rtnl_lock() > > > disable_nonboot_cpus() > > > > let's suppose disable_nonboot_cpus() does not take cpu_add_remove_lock, > > > > > -> flush CPU 3 workqueue > > > > in this case the deadlock is still here? > > > > We can't flush because we hold the lock (dpm_list_mtx) which depends > > on another lock taken by work->func(), the "classical" bug with flush. > > > > No? > > Yeah, it looks like cpu_add_remove_lock doesn't make a difference... > It's just lockdep reporting a longer chain that also leads to a > deadlock.
So. we should not call cpu_down/disable_nonboot_cpus under device_pm_lock().
At first glance this was changed by
PM: Change hibernation code ordering 4aecd6718939eb3c4145b248369b65f7483a8a02
PM: Change suspend code ordering 900af0d973856d6feb6fc088c2d0d3fde57707d3
commits. Rafael, could you take a look?
> OTOH just replace dpm_list_mtx with cpu_add_remove_lock and > you have the same scenario...
Yes, but
> happens too, I guess, somehow.
Oh, I hope not ;) nobody should use cpu_maps_update_begin() except cpu_down/up pathes. And workqueue.c, which uses it exactly because we want to call _cpu_down()->flush_cpu_workqueue() without any other locks held. But if the caller of cpu_down() holds some lock, then we have the usual problems with the flush under lock.
Oleg.
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